The importance of determining residual oil in place by means of subsurface logging techniques has been recognized for some time. Several techniques have been used in the past for this purpose. More recently, one of the more popular techniques measures the rate of thermal neutron decay following a burst of neutrons from a downhole generator. The rate of neutron decay is dependent upon the capture cross section of the formation rock, the capture cross section of the fluids contained within the formation rock, and the volumetric fractions of the rock and fluids. The measurements reflect only the total capture cross section of the composite material which is dependent upon many variables. If the contrast in the capture cross section between the oil and the formation waters is sufficient and the porosity of the formation is known, then qualitative evaluations of the water saturation can be performed.
This method is described in U.S. Pat. No. 3,562,523, issued Feb. 9, 1971, which patent is made a part of this disclosure.
A typical technique for determining residual oil using thermal neutron decay is known as log-inject-log. The principle of this technique is to inject water of a given salinity, either very high or very low, run a log, inject water at the other extreme salinity, and relog with the same tool. (By "logging" is meant measuring thermal neutron decay.) The only changes which should have taken place are in the salinity of the water in the borehole and in the formation surrounding the borehole. If the borehole effect is negligible, it is then possible to calculate the amount of water and oil in the formation. Present interpretation methods assume no borehole effect. However, there is some question as to the validity of this assumption. Our invention provides a means of eliminating, or substantially reducing, this borehole effect in the log-inject-log technique of measuring residual oil.
Briefly, our invention overcomes this borehole effect by adding to the low salinity water certain solid particles, as described hereinafter, which adjust the thermal neutron capture cross section of the well-bore so that it is approximately the same as the thermal neutron capture cross section of the well-bore when it contains the high salinity liquid.